A trashmob in DAI would be enemies like the bandits in the Hinterlands, Darksapwn on the Coast, Raiders in the Approach. Non boss enemies that, even though you clear the zone and kill all of, there are more next time you return to the zone. Those things some need to fill requisitions later in the game.
The majority of mobs do not respawn anyway once you finish a zone. Yes either Ventaroi or Red Templars will be everywhere and, will respawn with each visit to a zone and, a few others here and there but, not in droves like many games. For the most part a cleared zone remains a quiet zone.
I don't see the need for a no trash spawn mod in DAI, but if there is one, it would be at Nexus. if not then that would be the place to request such a mod. Just keep in mind that modding this game is not so easy and, it may not be possible without affecting something else.